As metal oxide semiconductor field effect transistor (MOSFET) feature sizes decrease, the gate oxide thickness of the devices also decreases. This decrease is driven in part by the demands of overall device scaling. As gate conductor widths decrease, for example, other device dimensions decrease to maintain the proper device scale, and thus device operation. Another factor driving reduction of the gate oxide thickness is the increased transistor drain current realized from a reduced gate dielectric thickness. The transistor drain current is proportional to the amount of charge induced in the transistor channel region by the voltage applied to the gate conductor. The amount of charge induced by a given voltage drop across the gate dielectric (e.g., the gate oxide) is a factor of the capacitance of the gate dielectric.
In order to achieve increased capacitance, gate oxide thicknesses have been decreased to as thin as 10 Å. These extremely thin gate oxides result in increased gate-to-channel leakage current, however. Problems such as this have led to the use of materials that have dielectric constants that are greater than the dielectric constant of silicon oxide, which has a k value of about 3.9. Higher k values, for example 20 or more, may be obtained with various transition metal oxides. These high-k materials allow high capacitances to be achieved with relatively thick dielectric layers. In this manner, the reliability problems associated with very thin dielectric layers can be avoided while improving transistor performance.
There are, however, fabrication problems associated with forming gate dielectric layers that include high-k materials, particularly when a metal gate is employed. For example, high dielectric materials may contain a greater number of bulk traps and interface traps than gate dielectrics made from thermally grown SiO2. Traps adversely affect both subthreshold slope and threshold voltage (Vt). High trap density also leads to leakage through Frenkel-Poole tunneling, and it causes bias temperature instability.
One class of high-k dielectrics that have received much attention recently is hafnium-based oxides. Unlike SiO2, wherein chemical bonding is predominately covalent, Hf-based oxides are predominately ionic and therefore exhibit their own host of problems. Control of flatband voltage (Vfb) has proven particularly difficult. Recent work has suggested that oxygen vacancy formation in the Hf dielectric and/or interfacial Hf reactions may account for the large observed Vfb, shifts, particularly in the case of p+ gates.
Therefore, there is a need for passivating materials, structures, and methods in the manufacture of semiconductor devices that use high-k dielectrics.